One aspect of a multi-access channel network is the resource allocation among the active users. As an example of a multi-access channel, the following scenario is considered: a point-to-multipoint network characterized by a base station (or a master station) that coordinates the communications among several terminal stations (or subscriber stations). Examples of point-to-multipoint networks are wireless access networks conforming to IEEE Standard 802.16, networks conforming to ETSI standards known as ETSI HiperLAN, ETSI HiperMAN, ETSI HiperAccess, and 3GPP networks. By way of example only, where necessary, reference will be made to IEEE Standard 802.16.
In such a scenario, the base station assigns time slots for each terminal station defining, in this way, the accesses to the multi-access channel. In the assigned slot, the terminal station transmits data with a specific modulation that can change in real time depending on the channel conditions: the transmission efficiency of each terminal station is bounded by the used modulation robustness. For instance, if the environmental conditions are good (e.g. short distance between the base station and a terminal station and line of sight), a terminal station can transmit with a more efficient modulation, sending a great amount of data in a short time slot. On the other hand, if a terminal station is far from the base station, the terminal station may transmit with a more robust modulation, at the expense of efficiency (e.g., it may take a relatively longer time to transmit relatively lesspayload information).
The operator analyzes and provides a territorial planr to define the maximum number of terminal stations that can be supported by the system and the maximum channel bit-rate.
Another aspect that the operator may take into account is the Quality of Service (QoS) associated with different classes of traffic. In fact, the sizes of the time slots assigned by the base station typically depend on the QoS parameters defined for each class of traffic. For instance, the above mentioned standard IEEE 802.16 specifies four classes of traffic, namely Unsolicited Grant Service, Real-time Polling Service, Non-real-time Polling Service and Best Effort, in decreasing order of QoS requirements, and hence in decreasing order of scheduling priority.
Each class of traffic is well known to the skilled in the art and can be found in the standard. Air time management is applicable to variable rate connections getting bandwidth on request, e.g. connections relevant to Real-time Polling services, Non-real-time Polling services and Best Effort services. The first two kinds of connections will also be denoted by the common term of “guaranteed bandwidth connections”.
In particular, for guaranteed bandwidth connections, two QoS parameters are specified: Minimum Reserved Traffic Rate (MRTR), which represents the guaranteed portion of data rate handled by the network, and Maximum Sustained Traffic Rate (MSTR), which represents the peak data rate. The MRTR portion is processed with the highest priority; the surplus portion (e.g. the difference between MSTR and MRTR) is served with lower priority, and only if remaining bandwidth is available.
For best effort connections, only the MSTR parameter is specified. Therefore, the traffic of best effort connections is treated as a surplus (equal to MSTR).
An air time management method for resource allocation to different terminals is disclosed in U.S. Pat. No. 6,564,047 B1, which is incorporated herein by reference. In order to coordinate access among active users, the time slots assigned by the base station are agreed and limited in accordance to the services each terminal station has to support. The limitation refers to the air time duration allocated to the terminal stations. Therefore, the terminal transmits data in the assigned slot, and the duration of the assigned slot is specified by the operator during the planning definition and is therefore fixed. The method is therefore not applicable to the case of adaptive modulation channels, as used in the above mentioned multi-access channel networks.
In adaptive modulation channels, modern planning methods adopted by operators take into account the channel maximum bit-rate associated with either the most robust modulation or the estimation of the mean modulation in order to define the maximum number of terminal stations supported by a cell.
A planning based on the most robust modulation represents the worst case. Such a planning assumes that all terminal stations always transmit with the most robust modulation and thus ensures that the channel can support the negotiated data traffic for all stations. However, this worst case scenario is not realistic (it is very unlikely that all terminal stations transmit with the most robust modulation) and is the result of an underestimation of the system efficiency. Consequently, a certain amount of resources will generally remain unused, because a number of terminal stations transmit with a more efficient modulation that consumes fewer resources, and there is a corresponding loss of revenues for the operator.
In a planning based on the estimation of the mean modulation, it is possible that some terminal stations will transmit with a less efficient (e.g. more robust) modulation than the expected one. For instance, a terminal station that is very close to the base station is expected to transmit with a very efficient modulation. If the above mentioned terminal station transmits with a more robust modulation, due to factors out of the control of the operator, e.g. a wrong antenna orientation or not line-of-sight positioning, the channel planning is not reliable and, unlike the first case, the operator overestimates the actual efficiency of the system. Since the channel is shared by all terminal stations, the scarcely efficient station steals bandwidth that, according to the planning, should have been allocated to other stations.